Effect of C132-stereochemistry on the molecular properties of chlorophylls

To elucidate the origin of differences in the monomeric properties of chlorophyll a and a' (Chl a/a'; C13(2)-(R/S)-epimers of Chl a), a series of Zn-Chl derivatives possessing a variety of C13(4) substituents were prepared. The (S)-epimers gave absorption and emission bands at slightly longer wavelengths than the corresponding (R)-epimers. The(S)-epimers had more intense CD peaks than the (R)-epimers. In comparison with Zn-Chl a/a' (C13(4)-Me), Zn-Chl with a bulky 2,4-dimethyl-3-pentyl moiety at the C13(4)-position showed a greater difference between the C-13 NMR chemical shifts of the epimers. These spectral differences between the (R)- and (S)-epimers were attributed to ruffling of the chlorin macrocycle, arising from a steric repulsion between the C13(2) and C17 moieties. The trend of the rate constant of epimerization, isomerization at the C13(2) position, in the C13(4)-substituted Zn-Chls is in line with the above interpretation. The relationship between the spectroscopic and kinetic results and the distortion of the macrocycle is schematically represented.